The present invention relates to a perpendicular magnetic recording medium, and more particularly, to a perpendicular magnetic recording medium capable of showing excellent magnetic properties, a high recording resolution and improved surface properties.
In recent years, in magnetic recording apparatuses, it has been more increasingly demanded to provide magnetic recording media having a high recording density property and a high reliability in order to deal with a large capacity data. Especially, in hard disk drives, there has been a remarkable tendency of a miniaturization and a high reliability in accordance with information devices being miniaturized and systems used therefor being required to have a high reliability.
In order to satisfy such properties, the magnetic recording media have been strongly required to have not only a high coercive force, but also a reduced distance (magnetic spacing) between a magnetic recording layer and a magnetic head.
As magnetic recording media having a high coercive force, there are widely known magnetic recording media comprising a substrate and a magnetic thin film formed on the substrate, in case of the hard disk drives.
The magnetic thin films already practically used in magnetic recording media, are generally classified into magnetic oxide thin films composed of maghemite or the like (“Technical Report of Electronic Telecommunication Institute”, published by Electronic Telecommunication Institute, (1981) MR81-20, pp. 5 to 12; “Ceramics”, published by Japan Institute of Ceramics, (1986) Vol. 24, No. 1, pp. 21 to 24; and Japanese Patent Publication (KOKOKU) Nos. 51-4086(1976) and 5-63925(1993)), and magnetic alloy thin films composed of Co—Cr based alloy or the like.
The magnetic oxide thin films composed of maghemite or the like are excellent in oxidation resistance or corrosion resistance due to inherent properties of oxides. As a result, the magnetic oxide thin films can show an excellent stability independent of the passage of time, and a less change in magnetic properties with the passage of time. Further, since the oxides exhibit a higher hardness as compared to metals, the magnetic oxide thin films do not require a protective layer to be formed thereon. Even if the protective layer is formed on the magnetic oxide films, the thickness of the protective layer can be reduced as compared to that required for magnetic alloy films. As a result, the magnetic oxide thin films can exhibit a smaller magnetic spacing than that of the magnetic alloy thin films. Therefore, the magnetic oxide thin films are most suitable for production of ultra-high density magnetic recording media.
The magnetic alloy thin films composed of Co—Cr based alloy or the like, have a coercive force as high as not less than 159 kA/m (2,000 Oe). However, these alloy materials themselves tend to be readily oxidized and, therefore, deteriorated in stability independent of the passage of time as well as magnetic properties with the passage of time.
In order to prevent the deterioration of magnetic properties due to the oxidation, the surface of the magnetic alloy thin film is coated with a protective layer composed of carbon, SiO2 or the like, which has a thickness of usually about 5 to 10 nm, resulting in undesired increase of magnetic spacing by the distance corresponding to the thickness of the protective layer.
On the other hand, in order to reduce the magnetic spacing of magnetic recording media, it is required to minimize the flying height of a magnetic head therefrom, and always allow the magnetic head to be flying stably. In conventional hard disk drives, magnetic recording media used therefor have been required to have a certain surface roughness in order to prevent the magnetic head from being absorbed thereon owing to a meniscus force therebetween upon stopping the magnetic head. However, as a result of current improvement in these hard disk systems, magnetic recording media have been no longer required to have such a surface roughness for preventing the magnetic head from being absorbed thereon. Also, it is known that the non-smooth surface of magnetic recording media causes media noise upon reproducing by the magnetic head. Therefore, in order to reduce such a media noise, the magnetic thin film used in the magnetic recording media is required to have a more excellent surface smoothness. In addition, with the decrease of the flying height of the magnetic head (distance between the media and the magnetic head), the thickness of the protective layer formed thereon tends to be reduced. For this reason, it has been further demanded that the magnetic recording layer in itself exhibits an excellent durability.
On the other hand, there are conventionally known two systems of magnetic recording methods, i.e., a perpendicular recording system and a longitudinal recording system. The perpendicular magnetic recording media are considered to be promising as future higher-density recording media, because of being less influenced by thermal fluctuation that will be caused upon achieving the ultra-high recording density (for example, “Nikkei Electronics”, Sep. 25, 2000, No. 779, etc.). That is, the perpendicular recording system is suitable for realizing the ultra-high recording density, since the influence of a demagnetizing field on residual magnetization of the media is lessened as a recording wavelength therefor becomes shorter.
Conventionally, there have been proposed magnetic recording media of a perpendicular recording system comprising a Co—Cr based perpendicular magnetic recording layer and a soft-magnetic (low-coercive force) backing layer such as a Ni—Fe based underlayer (hereinafter referred to merely as “soft-magnetic layer”) formed underneath the perpendicular magnetic recording layer. When such a soft-magnetic layer as an auxiliary magnetic pole is provided, it is possible to not only enhance the recording magnetic field produced by the single-pole magnetic head as a main magnetic pole, but also reduce a demagnetizing field of the magnetic recording layer after recording (Japanese Patent Application Laid-Open (KOKAI) No. 54-51804(1979), etc.).
As the perpendicular recording system, there have been proposed 1) a method of using a perpendicular magnetic layer provided with no soft-magnetic layer and a ring head; 2) a method of using a perpendicular magnetic layer provided with the soft-magnetic layer (perpendicular double-layered media) and a ring head; and 3) a method of using a perpendicular magnetic layer provided with the soft-magnetic layer and a single-pole magnetic head. An optimum method for realizing the ultra-high recording density is the above method 3) (for example, “Nikkei Electronics”, Jul. 1, 1996, No. 665, etc.).
The techniques using a perpendicular magnetic recording layer composed of spinel iron oxide such as maghemite are described in Japanese Patent Application Laid-Open (KOKAI) Nos. 11-110731(1999) and 11-110732(1999), etc. In order to obtain an excellent perpendicular magnetic recording layer, it is required to control a crystal orientation of a magnetic recording layer so as to preferentially orient a (400) plane of the spinel iron oxide layer in parallel with the surface of substrate by using a single-crystal substrate composed of MgO or NaCl, or providing an underlayer composed of NiO, MgO, Cr or the like, which has a thickness of 20 to 200 nm.
Also, in Japanese Patent Application Laid-Open (KOKAI) Nos. 6-168822(1994) and 11-339261(1999), it is described that a (400) plane of a maghemite thin film is oriented in parallel with the surface of a substrate, and a spacing of (400) plane of the maghemite thin film is controlled to not more than 0.2082 nm to induce a large magnetic anisotropy. More specifically, since a (200) plane of an NiO film used as an underlayer is oriented in parallel with the substrate and the spacing of the NiO film is 0.2089 nm which is larger than the inherent spacing of (400) plane of maghemite (0.2086 nm), a tensile stress is produced in the in-plane direction of the maghemite thin film, so that the spacing of (400) plane of the maghemite thin film formed on the NiO film is reduced to not more than 0.2082 nm, thereby exhibiting a large magnetostrictive anisotropy perpendicular to the film.
As to the magnetic recording media including a soft-magnetic layer formed between a substrate and a perpendicular magnetic recording layer composed of magnetic oxides such as maghemite, there are known the technique for controlling the orientation of a spinel iron oxide magnetic recording layer by forming an intermediate layer composed of non-magnetic metal such as Al between the soft-magnetic layer and the magnetic recording layer (Japanese Patent Application Laid-Open (KOKAI) No. 59-157828(1984)); the magnetic recording medium having a magnetic layer composed of iron oxide which is formed on the substrate directly or through a high-permeability magnetic thin film (Japanese Patent Application Laid-Open (KOKAI) No. 60-95721(1985)); or the technique for forming as the soft magnetic layer, a spinel iron oxide soft-magnetic thin film having the same crystal structure as that of maghemite, and further forming an underlayer for controlling the crystal orientation between the substrate and the soft-magnetic layer (Japanese Patent Application Laid-Open (KOKAI) No. 6-44550(1994), etc.).
Further, there are known techniques for producing excellent perpendicular magnetic recording media by controlling the ratio of a residual magnetization (Mr//) obtained when magnetized in the in-plane direction to a residual magnetization (Mr⊥) obtained when magnetized in the perpendicular direction, to a specific value (Japanese Patent Application Laid-Open (KOKAI) Nos. 59-157828(1984), 59-157829(1984), 59-157830(1984), 59-157833(1984), 59-157838(1984) and 60-95721(1985), etc.).
At present, it has been strongly required to provide a perpendicular magnetic recording medium comprising a maghemite thin film not only having an excellent recording resolution, a high coercive force and a high squareness but also an excellent surface smoothness which is also capable of minimizing an in-plane magnetic component in order to reduce noises due to the magnetic recording medium itself. However, the conventional perpendicular magnetic recording media have failed to satisfy these properties.
That is, in the method described in Japanese Patent Application Laid-Open (KOKAI) No. 59-157828(1984), the orientation of the spinel iron oxide magnetic recording layer is controlled by forming the intermediate layer composed of non-magnetic metal such as Al between the soft-magnetic layer and the magnetic recording layer. However, since the spacing of (200) plane of Al is narrower than that of (400) plane of the spinel iron oxide, it may be difficult to induce a large perpendicular magnetic anisotropy due to inverse magnetostrictive effect. In addition, since the intermediate layer has a thickness as large as from 10 nm to 5 μm, a magnetic spacing between the magnetic head and the soft-magnetic layer upon recording becomes large, thereby inhibiting a good magnetic interaction between the magnetic head as a main magnetic pole and the soft-magnetic backing layer as an auxiliary magnetic pole.
Also, in Japanese Patent Application Laid-Open (KOKAI) No. 60-95721(1985), it is described that a high-permeability thin film having a thickness of 0.2 to 2.0 μm and a magnetic recording layer are successively formed on a substrate. However, this KOKAI is silent about controlling the crystal orientation by forming a non-magnetic layer, etc., between the high-permeability thin film and the magnetic recording layer or between the high-permeability thin film and the substrate, and describes neither crystal orientation nor surface properties of the magnetic recording layer.
Further, in the case where the spinel iron oxide soft-magnetic layer described in Japanese Patent Application Laid-Open (KOKAI) No. 6-44550(1994) is provided, since the soft-magnetic ferrite layer having a spinel structure exhibits a large lattice constant as compared to that of spinel iron oxide used for the magnetic recording layer, a tensile stress is exerted on the magnetic recording layer, so that it is possible to induce a large perpendicular magnetic anisotropy due to inverse magnetostrictive effect. However, in order to form the spinel oxide soft-magnetic layer having an excellent soft-magnetic property, it is required to use a substrate temperature as high as about 400° C. and a heat-treating temperature as high as about 550° C. upon forming the film, thereby failing to obtain a perpendicular recording medium having an excellent surface smoothness.
Further, in Japanese Patent Application Laid-Open (KOKAI) Nos. 59-157828(1984), 59-157829(1984), 59-157830(1984), 59-157833(1984) and 59-157838(1984), etc., there is described the ratio of a residual magnetization (Mr//) obtained when magnetized in the in-plane direction to a residual magnetization (Mr⊥) obtained when magnetized in the perpendicular direction. However, since the ratio is not more than 2, i.e., the in-plane magnetic component is large, it may be difficult to sufficiently reduce the media noises.
In addition, in Japanese Patent Application Laid-Open (KOKAI) No. 60-95721(1985), relating to the perpendicular magnetic recording medium composed of spinel iron oxide, there are described the ratio of a residual magnetization (Mr//) obtained when magnetized in the in-plane direction to a residual magnetization (Mr⊥) obtained when magnetized in the perpendicular direction, and the ratio of a coercive force (HC//) obtained when magnetized in the in-plane direction to a coercive force (Hc⊥) obtained when magnetized in the perpendicular direction. Although the increase of in-plane magnetic component is inhibited, the obtained perpendicular magnetic recording medium has a coercive force as low as 151 kA/m (1,900 Oe), thereby failing to provide a ultra-high density perpendicular recording medium as required.
Under the circumstances, as a result of the present inventors' earnest studies for solving the above problems, it has been found that by successively forming either a soft-magnetic layer, an NaCl-type oxide layer for orientation control and a magnetite thin film, or an NaCl-type oxide layer for orientation control and a magnetite thin film, on a substrate by a sputtering method, and then transforming the magnetite thin film into a maghemite thin film, the obtained perpendicular magnetic recording medium including the maghemite thin film can show not only an excellent recording resolution, a high coercive force and a high squareness, but also an excellent surface smoothness, and is also capable of minimizing an in-plane magnetic component in order to reduce noises due to the magnetic recording medium itself. The present invention has been attained based on the above finding.